Preheat oven to 400 degrees and place cast iron skillet in oven while heating.
Meanwhile melt 4 tbsp. of butter over low heat on stove top.
Remove cast iron skillet from oven in 8-10 minutes.
Add 1 tbsp. of butter to hot pan and swirl around. Set aside.
Then reduce oven temperature to 350 degrees.
Whisk together dry ingredients in large mixing bowl.
In small separate bowl add milk, eggs, slightly cooled butter, & optional vinegar and whisk together.
Pour wet ingredients into bowl with dry and stir till incorporated.
Pour batter into preheated buttered skilled and bake 20-25 minutes until toothpick inserted comes out clean.